Charge density wave and crystalline electric field effects in TmNiC₂

Abstract

Single crystals of TmNiC₂ were grown by the optical floating-zone technique and were investigated by x-ray diffraction (XRD), thermal expansion, electrical resistivity, specific heat, and magnetic susceptibility measurements. Single-crystal XRD reveals the formation of a commensurate charge density wave (CDW) characterized by a CDW modulation vector q₂c=(0.5,0.5,0.5), which is accompanied by a symmetry change from the orthorhombic space group Amm2 to the monoclinic space group Cm, i.e., to a CDW superstructure which is isostructural with that of LuNiC₂. For all transport and thermodynamic properties, anomalies related to a second order-type thermodynamic CDW phase transition are observed at around TCDW≃375K. The large specific heat anomaly at TCDW, ΔC≃6.2Jmol⁻¹K⁻¹, together with noticeable changes in entropy and enthalpy related to the CDW transition, suggests that this point group symmetry breaking CDW phase transition affects more significant parts of the Fermi surface as compared to the incommensurate CDW transition of, e.g., SmNiC₂ with no change in point group symmetry. The results on the antiferromagnetic and paramagnetic state of TmNiC₂ obtained by the above macroscopic techniques were complemented by microscopic studies via inelastic neutron scattering. A crystalline electric field modeling of macroscopic susceptibility and magnetic specific heat and entropy contributions as well as microscopic neutron scattering data, reveal crystal field eigenstates and eigenvalues with a ground-state doublet of the Tm-4f electrons, which is well separated by about 25 meV from exited states of the J=6 ground-state multiplet.